This application is based on Application No. 2000-380504, filed in Japan on Dec. 14, 2000, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a cell shunt (charging current shunt) circuit provided as overcharge protective measures for a plurality of individual lithium ion battery cells connected in series with one another to constitute a lithium ion battery for example, which can be installed on a satellite, spacecraft, etc.
2. Description of the Related Art
FIG. 4 illustrates a known cell shunt for a lithium ion battery cell. The known cell shunt includes a plurality of lithium ion battery cells C1-Cn, and a plurality of cell shunt circuit sections Sh1-Shn, and a battery charger 8. Each of the cell shunt circuit sections Sh1-Shn is constituted by a shunt transistor 4, a reference voltage generating section 5, and a differential amplifier 6. In addition, a symbol Ichg represents a battery charging current, Ip a shunt current, Vc a cell voltage, and Vs a reference voltage generated by the reference voltage generating section 5.
In a nickel cadmium battery and a nickel hydrogen battery used in the past as batteries for a satellite and a spacecraft, a plurality of battery cells connected in series with one another are charged by performing batch constant-current charging in order to achieve reduction in resources of a battery charger and improvements in reliability. On the other hand, a lithium ion battery cell, which has high energy density and various excellent characteristics for installation on a satellite and a spacecraft, is becoming the mainstream of future satellite and spacecraft batteries.
In such a batch constant-current charging method, however, variations in individual capacities of the battery cells result in variations in the charging amounts for the respective cells, so that some battery cells may be overcharged.
In particular, lithium ion battery cells have a characteristic that the lifetime property is remarkably decreased due to overcharging. This is a weak point for satellite and spacecraft batteries for which a long life time is required. Thus, the cell shunt for lithium ion battery cells as referred to above is required in order to cope with such a problem.
The known cell shunt for lithium ion battery cells is constructed as shown in FIG. 4. In this known cell shunt, a battery charging current Ichg is supplied to the lithium ion battery cells C1-Cn connected in series with the battery charger 8 to charge the lithium ion battery cells C1-Cn in a batch processing manner. By this charging, charge energy is uniformly accumulated in the respective lithium ion battery cells C1-Cn in proportion to the product of the battery charging current Ichg and a charging current time or duration. In this process, a cell voltage Vc in each of the lithium ion battery cells C1-Cn rises according to a charge energy limit of each cell (i.e., variations in the capacities of the cells). Although a cell having the smallest capacity first reaches a charge completion voltage, charging is continued until all the lithium ion battery cells C1-Cn reach the charge completion voltage.
In this charging operation, when a lithium ion battery cell C1 first reaches the charge completion voltage for instance, the differential amplifier 6 in the cell shunt circuit section Sh1 detects that a voltage Vc across the cell C1 reaches a reference voltage Vs which is preset to a value equal to the charge completion voltage, and drives the shunt transistor 4, so that a surplus current (hereinafter, called a shunt current Ip) is shunted from the battery charging current Ichg, thus preventing the battery charging current Ichg from being supplied to the lithium ion battery cell C1. The above-mentioned operation is similarly performed in each of the cell shunt circuit sections Sh1-Shn, whereby each of the lithium ion battery cells C1-Cn is prevented from being overcharged in the continued charging operation.
In the cell shunt circuit for lithium ion battery cells as described above, by shunting the shunt current Ip from the battery charging current Ichg through each shunt transistor 4, the lithium ion battery cells C1-Cn are prevented from being overcharged. However, the shunt current Ip flows through each shunt transistor 4 to generate heat P therein, as expressed by the following equation (1).
P=Ichgxc3x97Vcxe2x80x83xe2x80x83(1)
This heat P makes the thermal design of the satellite and spacecraft battery system difficult. Moreover, it is a factor of disturbing the reduction in resources and the improvement in reliability of the battery charger.
The present invention is intended to solve above-mentioned drawbacks, and the object of the present invention is to suppress the generation of heat in a cell shunt circuit for battery cells.
Bearing the above object in mind, the present invention resided in a cell shunt circuit for battery cells including a plurality of shunt circuit sections connected in parallel with the battery cells, respectively, which are connected in series with one another so as to be charged in a batch manner by a battery charger, each of the cell shunt circuit sections comprising: an energy reservoir acting as a bypass path for bypassing a charging current supplied to a corresponding one of the battery cells so as to be input to the following battery cell provided at a downstream side of the one battery cell so as to reserve surplus energy obtained from the charging current thus bypassed and regenerate the thus reserved surplus energy to a batch charging line connected with the serially connected battery cells; a switching element inserted in the bypass path for opening and closing thereof; and a comparator for making a comparison between a charging voltage across the corresponding one of the battery cells and a reference voltage and outputting a driving signal to the switching element when the charging voltage is greater than the reference voltage; wherein the energy reservoir regenerates the surplus energy to the batch charging line when the switching element opens the by-pass path.
In a preferred form of the present invention, each of the shunt circuit sections further comprises a low-pass filter for detecting a charging voltage of the corresponding one of the battery cells with a response time delay and outputting a detection output to the comparator.
In another preferred form of the present invention, the battery charger comprises: a power conversion section for converting a direct current input from an exterior into a direct current; and a current control section for outputting a constant direct current to the battery cells based on an output of the power conversion section, the surplus energy being regenerated from the energy reservoir into the current control section in the same direction as that of the output from the power conversion section.
In a further preferred form of the present invention, the energy reservoir comprises a flyback transformer having a primary winding and a secondary winding which are connected in such a manner that the charging current of the battery cells is bypassed to the primary winding of the flyback transformer, and that an output of the secondary winding of the flyback transformer is regenerated to the batch charging line, with a diode being connected with the secondary winding in a direction to permit the output of the secondary winding to be supplied to the batch charging line.
The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of preferred embodiments of the present invention taken in conjunction with the accompanying drawings.